1. Field of the Invention
The present invention relates to the use of coherent energy pulses, as from high powered pulsed lasers, as well as an ultraviolet curable resin and an ultraviolet light in the shock processing of materials, and, more particularly, to methods and apparatus for improving properties of solid materials by providing shock waves therein. The invention is especially useful for enhancing or creating desired physical properties such as hardness, strength, and fatigue strength.
2. Description of the Related Art
Laser peening (hereinafter referred to as laser shock processing) utilizes two overlays, a transparent overlay (usually water), and an opaque overlay (previously a black paint). During processing, a laser is directed to pass through the water overlay and is absorbed by the black paint, causing a rapid vaporization of the paint surface and the generation of a high-amplitude shock wave. The shock wave cold works the surface of the part creating compressive residual stresses, which provide an increase in fatigue properties of the part. A workpiece is typically processed by processing a matrix of overlapping spots that cover fatigue critical zones of the part.
The current art utilizing laser shock processing has a problem with the amount of time necessary to dry the black paint. The problem with drying the black paint usually occurs with solvent-based paint but other types of paint may have this problem. Another problem is that the black paint begins eroding as soon as the water layer is applied to the paint. This problem with erosion usually occurs with water-based paint but other types of paint may have this problem. During the laser processing of a workpiece, the black paint must be applied to the workpiece multiple times. Each coat of paint takes approximately ten minutes to dry, which makes the processing time lengthy. Also, once the transparent overlay is applied to the paint, the paint may start to erode immediately. This opaque layer erosion causes a turbulence in the surface of the workpiece and reduces the shock pressure during the laser shock processing cycle. Therefore, the paint must be applied again to the workpiece further increasing the processing time. This problem with erosion of the paint decreases the efficiency and effectiveness during the laser shock processing of the workpiece. The reasons for the efficiency and effectiveness being decreased is because of the amount of time the paint takes to dry and because the paint begins eroding immediately once the transparent overlay is applied which causes the shock pressure to the workpiece to be reduced once the laser is applied to the eroding paint.
The present invention provides a method of laser shock processing that can be used in a production environment that significantly reduces laser shock processing time and increases the effectiveness of the laser shock processing of the workpiece. The method begins with a step of applying an energy absorbing coating to the surface of the workpiece that is to be laser shock processed. In one form of the invention, an ultraviolet-curable resin, such as an ultraviolet-curing acrylic or urethane resin, is applied to the energy absorbing coating of the workpiece. The present invention is not necessarily limited to ultraviolet-curable resins, any type of curable resin can be used. A resin is cured when the resin becomes chemically inert or upon the polymerization of the resin. In one form, the ultraviolet-curable resin is exposed to an ultraviolet light which causes the curable resin to form a pellicle over the energy absorbing coating of the workpiece. The present invention is not limited to an ultraviolet light to form a pellicle or skin over the resin. The curing effect is determined by the type of resin used and the means of curing that resin. A transparent overlay, such as water, is applied in a thin flowing layer over the pellicular energy absorbing layer. When the transparent overlay has covered the energy absorbing layer of the workpiece, the laser is fired through the transparent overlay and onto the energy absorbing coating. After the laser has been fired, the remaining portion of the coating can be washed off of the surface of the workpiece using a high-speed jet of fluid. The entire sequence and event timing is controlled by an electronic processor. The sequence is repeated for each spot to be processed along the workpiece surface.
The invention, in one form thereof, comprises a method of laser shock peening the surface of a solid material. An energy absorbing coating is applied to a portion of the surface of the solid material. An ultraviolet-curable resin is applied to the coated portion of the solid material and the curable resin is exposed to an ultraviolet light and forms a pellicle over the energy absorbing coating. A transparent overlay material is applied to the pellicular portion of the energy absorbing coating. A pulse of coherent energy is directed to the energy absorbing coating of the solid material to create a shockwave.
The invention, in another form thereof, comprises a method of laser shock peening the surface of a solid material with or without the use of a transparent overlay material. An energy absorbing coating is applied to a portion of the surface of a solid material. An ultraviolet-curable resin coating is applied to the energy absorbing coating and the curable resin is exposed to an ultraviolet light and forms a pellicle over the energy absorbing coating. A pulse of coherent energy is directed to the energy absorbing coating of the solid material to create a shock wave.
The invention, in yet another form thereof, comprises a method of laser shock peening the surface of a solid material. An energy absorbing material is mixed with an ultraviolet-curable resin to form an energy absorbing mixture coating that is applied to the surface of the solid material. The mixture coating is exposed to an ultraviolet light and forms a pellicle over the mixture coating. A transparent overlay material is applied to the pellicular portion of the energy absorbing coating and a pulse of coherent energy is directed to the energy absorbing coated portion of the solid material to create a shockwave.
The invention, in still another form thereof, comprises a method of laser shock peening the surface of a solid material without the use of a transparent overlay material and using a mixture of the resin and the energy absorbing material. An energy absorbing material is mixed with an ultraviolet-curable resin to form an energy absorbing mixture coating that is applied to the surface of the solid material. The mixture coating is exposed to an ultraviolet light and forms a pellicle over the mixture coating and a pulse of coherent energy is directed to the energy absorbing coated portion of the solid material to create a shock wave.
The invention, in yet another form thereof, comprises a method of laser shock peening the surface of a solid material using a mixture consisting of a wet energy absorbing material and a resin. A wet energy absorbing material is mixed with an ultraviolet-curable resin to form an energy absorbing coating that is applied to the surface of the workpiece. The curable resin causes the wet energy absorbing coating to form a pellicle over once exposed to an ultraviolet light. A pulse of coherent energy is directed to the energy absorbing coated portion of the solid material to create a shock wave.
The invention, in still another form thereof, comprises an apparatus for laser shock peening the surface of a solid material. The apparatus includes a material applicator for applying an energy absorbing material onto the workpiece to create a coating on the workpiece. A curable resin applicator applies an ultraviolet curable resin onto the energy absorbing coating of the workpiece and a light applicator applies an ultraviolet light to the curable resin. A transparent overlay applicator applies a transparent overlay to the energy absorbing coating of the workpiece. A laser provides a laser beam through the transparent overlay and to the energy absorbing coating to create a shock wave on the workpiece. A positioning mechanism is included to selectively position the workpiece relative to the material applicator, the curable resin applicator, the light applicator, the transparent overlay applicator, and the laser. Conversely, a positioning mechanism may position the material applicator, the curable resin applicator, the light applicator, and the transparent overlay applicator correctly over the spot on the workpiece to be treated while it is in position in the laser beam path. A control unit is operatively associated with each of the applicators, laser, and positioning mechanism to control the operation and the timing of each of the applicators, laser, and the selective operation of the positioning mechanism.
The invention, in yet another form thereof, comprises an apparatus for laser shock peening the surface of a solid material. The apparatus includes a material applicator for applying an energy absorbing material onto the workpiece, to create a coating on the workpiece. An ultraviolet curable resin applicator applies a curable resin to the energy absorbing coated portion of the workpiece and an ultraviolet light applicator applies an ultraviolet light to the curable resin. A laser provides a laser beam to the energy absorbing layer to create a shock wave on the workpiece. A positioning mechanism is included to selectively position the workpiece relative to the material applicator, curable resin applicator, light applicator, and the laser. Conversely, the positioning mechanism may position the material applicator, light applicator, and curable resin applicator correctly over the spot on the workpiece to be treated while it is in position in the laser beam path. A control unit is operatively associated with each of the applicators, laser, and positioning mechanism, to control the operation and the timing of each of the applicators, laser, and the selective operation of the positioning mechanism.
The invention, in still another form thereof, comprises an apparatus for laser shock peening the surface of a solid material. The apparatus includes a material applicator for applying a mixture of an energy absorbing material and an ultraviolet curable resin onto the workpiece to create a coated portion and a light applicator applies an ultraviolet light to the mixture. A transparent overlay applicator applies a transparent overlay to the energy absorbing mixture coating of the workpiece. A laser provides a laser beam through the transparent overlay and to the energy absorbing coating to create a shockwave on the workpiece. A positioning mechanism is included to selectively position the workpiece relative to the material applicator, light applicator, the transparent overlay applicator, and the laser. Conversely, a positioning mechanism may position the material applicator, the light applicator, and the transparent overlay applicator directly over the spot on the workpiece to be treated while it is in position in the laser beam path. A control unit is operatively associated with each of the applicators, laser, and positioning mechanism to control the operation and the timing of each of the applicators, laser, and the selective operation of the positioning mechanism.
The invention, in yet another form thereof, comprises an apparatus for laser shock peening the surface of a solid material. The apparatus includes a material applicator for applying a mixture of an energy absorbing material and an ultraviolet curable resin onto the workpiece to create a coated portion and a light applicator applies an ultraviolet light to the mixture. A laser provides a laser beam through the energy absorbing coating to create a shock wave on the workpiece. A positioning mechanism is included to position the workpiece relative to the material applicator, light applicator, and the laser. Conversely, the positioning mechanism may position the material applicator and light applicator over the spot on the workpiece to be treated while it is in position in the laser beam path. A control unit is operatively associated with each of the applicators, laser, and positioning mechanism to control the operation and the timing of the applicators, laser, and the selective operation of the positioning mechanism.
An advantage of the present invention is that the method assists in erosion resistance of the energy absorbing coating once the transparent overlay is applied because the curable resin forms a pellicle over the energy absorbing coating and therefore delaying the erosion of the coating from the transparent overlay material. Prior laser shock processing methods have a problem in which, once the transparent overlay is applied to the energy absorbing coating, erosion of the energy absorbing coating takes place immediately and the coating has to be applied again. Having to re-apply the coating significantly increases the processing time.
Another advantage of the present invention is that the curable resin will form a pellicle over the energy absorbing coating in less than one second. Normally, it takes approximately ten minutes for the energy absorbing coating to dry so that laser shock processing of the workpiece can occur. Using the fast curing resin, the wet paint is protected in less than one second, which increases the processing time significantly.
A further advantage of the present invention is that higher peak pressures of the shock wave to the workpiece can be achieved because the curable resin causes more of its energy absorbing coating to be left intact when the laser beam impacts the energy absorbing layer. Eliminating premature mixing of the energy absorbing coating with the transparent overlay allows for higher peak pressures of shock waves to the workpiece.
Yet another advantage of the present invention is that reduced power can now be used to achieve the same residual stress levels because previously, higher power was used to compensate for the erosion of the energy absorbing layer. Due to the fact that the curable resin assists in the resistance of the erosion to the energy absorbing layer, less power can be used to achieve the same results which will increase the life of the laser equipment.
Another advantage of the present invention is that by the curable resin forming a pellicle rather than becoming fully cured, the energy absorbing coating is more easily cleaned off the workpiece once the laser beam has been applied.